Customers will eventually replace their gasoline-powered vehicles with rechargeable models. Accordingly to a McKinsey report by 2015, approximately 2 million electric vehicles (EVs) will be sold around the world. This replacement will lead to substantial decrease in carbon footprint. For instance, the replacement of 77% of all transport miles with EVs will reduce carbon intensity by 94% over the 1990 numbers. To meet this demand, automotive companies are taking tremendous efforts on enhancing battery technologies including battery cell manufacturing and pack manufacturing. In particular, in battery pack manufacturing, battery management is essential so as to make the performance of rechargeable batteries competitive with, and attractive alternatives to, conventional gasoline engines.
To improve battery performance, battery management needs 1) cell balancing and 2) the prevention of the overcharge and deep-discharge of battery cells. Since characteristics of individual battery cells are not the same, strong cells can put stress on weak cells, and vice versa. Cell balancing is also very important for battery health. On the other hand, when a lithium-ion cell (that has high electrical energy concentrated in a small volume in the cell) is overcharged, active materials therein will most likely react with other materials and electrolytes, potentially causing an explosion, let alone damaging the cell itself. When the cell is deep-discharged, or it continues to be discharged, despite its terminal voltage below a certain threshold called the cutoff voltage, it may become short-circuited, transitioning the cell into an irreversible condition. When the cells are connected in parallel it is important to balance their voltages, since their interactions and dependencies make their voltages drift apart. Higher-voltage cells may then inversely charge the lower-voltage cells, causing the entire terminal voltage to drop from the desired value of the parallel—connected cells. Moreover, a lithium-ion cell has unique characteristics, such as discharge efficiency (the higher the discharge rate, the lower the deliverable capacity), and recovery efficiency (the interface-concentrated gradient inside the cell is diffused during a ‘rest,’ after which the cell can be charged with large electric current over a short time).
Given series-connected cells, various methods for cell—balancing exist. First, cells with a higher state-of-charge (SoC) than others use up their redundant energy. For instance, when there are cell A and B each that have 60% and 50% SoCs, cell A burns 10% of its SoC to equalize SoC. Although this method is undesirable from the energy efficiency perspective, it has been adopted in industries due to the simplicity of the implementation of it. Alternatively, cells with a higher SoC are used to charge cells with a lower SoC. The redundant energy is stored in an inverter, and then the inverter is used to charge cells with a lower SoC. In this method, energy loss is important to minimize in converting DC to AC and vice versa.
Given parallel-connected cells, we can schedule their charge, discharge, and rest activities. For instance, each cell can be discharged in a round-robin fashion. Furthermore, the amount of discharge time can be scheduled in proportion to the remaining charge current in the corresponding cell, indicated by the state-of-charge (SoC) level. In general, however, no single mechanism outperforms the others in all circumstances, thus calling for a thorough study of this issue.
Three main challenges exist in scheduling charge, discharge, and rest activities as well as cell-balancing, for large-scale battery systems. First, a scheduling framework should operate reasonably well in all circumstances. That is, using the framework, one should be able to extend a battery cell's operation-time as much as any other scheduling mechanism can. By ‘operation-time,’ we mean the cumulative time of the charge current drawn from a battery cell until the cell no longer delivers the required charge current to applications. That is, the operation-time ends when the terminal voltage of the cell falls below the cutoff voltage. To extend the cell's operation-time, we need to understand the battery characteristics, such as the discharge and recovery efficiency.
Second, a scheduling framework should be robust to (inevitable) cell failures in a large-scale battery pack in which cells interact with, and depend on, each other. The terminal voltage of a weak cell with low capacity tends to drop quicker than other cells in use. The voltages of all cells (including the weak one) must remain balanced. When a weak cell cannot reach the full charge owing to high self-discharge, and/or becomes short-circuited, healthy cells could be overcharged.
Third, series-connected battery cells are important to balance while overhead, e.g., energy efficiency, for cell balancing is kept minimum. Cell balancing should not only keep the battery cells healthy, but also extend their operation-time, in which significant overhead is undesirable.
This section provides background information related to the present disclosure which is not necessarily prior art.